The present invention relates to composites of porous materials infiltrated with a second composition and to processes for their manufacture. More particularly the invention relates to processes to fully infiltrate porous ceramic materials with partially wetting compositions and the composites made thereby. In particular, the invention relates to fully infiltrated ceramic-metal composites and their method of manufacture.
Great strides in the development of ceramic-metal composite materials have been made over the past three decades. Ceramic-metal composites including metal-reinforced ceramic matrix composites (CMC), also known as cermets, and ceramic-reinforced metal matrix composites (MMC) blend the properties of hardness, rigidity, heat resistance and chemical resistance associated with ceramics, with properties such as electrical conductivity, mechanical strength, toughness and thermal shock resistance associated with metals. Such composites are useful in the manufacture of electrodes for molten electrolyte systems, components for internal combustion, jet and rocket engines, valves, armaments, armor and equipment for cutting, drilling, grinding and crushing. A preferred method of producing ceramic-metal composites is to infiltrate a porous ceramic particulate or fiber pre-form with a metal. Ceramic preforms may be easily machined into desired shapes prior to metal infiltration and densification by infiltration occurs with little or no shrinkage. Infiltration processes also tend to have the added advantage of reduced cycle times compared to other methods of ceramic-metal composite production.
One of the present difficulties in ceramic-metal composite production is the incomplete infiltration of the molten metal into the porous infrastructure of the ceramic material. This is particularly true for infiltration into spaces larger than the close-packed inter-particle spaces, such as unintentional packing defects or intentionally designed pores intended to provide large metal phases imbedded in the ceramic material. Typically, liquid metals do not wet ceramics well enough for spontaneous infiltration to occur. Even when the contact angle, xcex8, for a given combination of molten metal and ceramic is less than 90 degrees, making the system partially wetting, the liquid will still not necessarily spontaneously infiltrate due to non-cylindrical pore shapes. For example, the threshold contact angle for spontaneous infiltration of close-packed spheres is 50.7 degrees. Furthermore, even when the contact angle is low enough to enable spontaneous infiltration into the close-packed structure, large pores that are several times the size of the pores between the packed particles are not filled, again due to non-cylindrical pore shape effects.
The prior art describes two subclasses of infiltration processes known as xe2x80x9cforcedxe2x80x9d or xe2x80x9cpressurexe2x80x9d infiltration (PI), and spontaneous infiltration (SI). Ceramic-metal combinations, which are non-wetting, must use PI to affect infiltration. Such non-wetting systems are characterized by a contact angle, xcex8, larger than 90xc2x0 for a liquid metal drop resting on a flat ceramic substrate surface. In such systems, the replacement of solid-vapor surface area with solid-liquid surface area (i.e., infiltration) increases the surface free energy of the system and so requires the positive pressures of PI to force molten metal into the pores of the ceramic material against a positive capillary pressure and to prevent the metal from wicking back to the surface before the metal solidifies. PI processes typically require pressures in the range of 0.5 MPa to 170 MPa (1 MPa is 1 mega-pascal=9.869 atmospheres=7500.6 Torr); however, the lower pressures tend to lead to incomplete pore filling even in uncommonly large pores between ceramic fibers packed to only 24% density. The higher pressures are required to obtain more complete infiltration of common industrial ceramics. Such high pressures can damage the ceramic preforms and require costly production equipment such as mechanical (hydraulic) presses with heated dies and high pressure vessels able to exert the pressure by gas.
Ceramic-metal systems that are xe2x80x9cwettingxe2x80x9d are thought to be suitable for SI. Wetting systems are characterized by contact angles, xcex8, of less than 90xc2x0, though typically spontaneous infiltration only occurs with much lower contact angles, say below about 60xc2x0, due to its dependency on pore geometry. In such systems, the replacement of solid-vapor surface area with solid-liquid surface area (i.e., infiltration) decreases the surface free energy of the system. Molten metal thus imbibes spontaneously, provided there are no energy barriers preventing the attainment of the lower energy state.
Real systems, however, are characterized by irregular local pore geometries, which may present energy barriers preventing or limiting the extent of infiltration. Thus, even with wetting systems, PI is often necessary to achieve more complete infiltration. A number of methods have been developed to enhance wetting and infiltration of metal(s) into ceramic materials. These include the addition of wetting agents or reacting either the metal or ceramic prior to infiltration to facilitate wetting. See, for example, U.S. Pat. No. 3,864,154 to Gazza et al., describing the addition of silicon to aluminum as a wetting agent to infiltrate a silicon boride or aluminum boride or boron ceramic; U.S. Pat. No. 3,718,441 to Landingham, in which the metal is heated at low pressure to remove an oxide film to facilitate wetting; U.S. Pat. No. 4,617,053 to Joo et al. which teaches the chemical modification of the pore surfaces of a boron carbide ceramic to facilitate the wetting of reactive metals such as aluminum and aluminum alloys. These methods for facilitating SI are based on the concept of reducing the contact angle between the molten metal and ceramic surface. These methods still do not solve the problem of infiltrating large pores and defects within the ceramic material.
Vacuum infiltration methods have also been developed to minimize voids in ceramic-metal composites. The goal of these methods has been to prevent the entrapment of gas within the ceramic material that would prevent complete infiltration. See, for example, Toy and Scott, Ceramic-Metal Composite Produced by Melt Infiltration, Journal of American Ceram. Soc., 73 (1) 97-101 (1990). Under these techniques, a ceramic material is embedded in or surrounded by the infiltrant metal and placed in a furnace. The furnace is then evacuated to a set pressure, heated to melt the metal, which leads to infiltration, and then cooled to solidify the metal while maintaining the vacuum pressure. Though these vacuum techniques tend to improve infiltration, capillary effects still prevent spontaneous infiltration of larger pores and defects within the ceramic, resulting in voids within the ceramic-metal composite.
There remains a need for general techniques to improve infiltration of metal compositions into porous ceramics under simplified conditions that do not require expensive and bulky high-pressure equipment. In addition, there remains a need for improved processes of producing ceramic-metal composites without the use of high pressures that would damage fine pre-form structures.
It is an object of this invention to provide processes for producing composites from wetting systems without the need for high-pressure furnaces. It is further an object of this invention to provide processes of composite production that reliably provide complete infiltration of liquid phase material into porous solids including infiltration of large pores and defects in the porous substrate. Another object of the present invention is to provide processes to fully infiltrate ceramic materials with molten metal compositions to produce ceramic-metal composites. A further object of the invention is to provide a generalized process for determining optimum process conditions for the complete infiltration of any given porous ceramic material by any given metal composition.
Further objects, embodiments and features of the present invention will be apparent from the following description and the accompanying figures and table.